During the past six months, the Internet engineering community has undertaken an unprecedented effort to promote IPv6 as an urgent and necessary upgrade for network and website operators to allow for the continued, rapid growth of the Internet.

The symbol of that effort is World IPv6 Day, a 24-hour trial of IPv6 that is sponsored by the Internet Society and scheduled for June 8. So far, 200-plus website operators -- including Google, Yahoo and Facebook -- have agreed to participate in the event by serving up their content via IPv6, an upgrade to the Internet's main communications protocol called IPv4.

Amid the buzz around World IPv6 Day, it's hard not to wonder: What if IPv6 fails to catch on after this event? What if the Internet is still 99 percent based on IPv4 five years from now as it is today?

Proponents of IPv6 make dire predictions about the fate of the Internet if usage of IPv6 doesn't rise dramatically in the next few years. They say the complexity of the Internet infrastructure will increase, network operations costs will rise, and innovation will be hampered. This is due to the multiple layers of network address translation (NAT) devices that will be required to share limited IPv4 addresses among a rapidly growing base of users and devices.

"If IPv6 fails to catch on, then the Internet will include nesting of NAT upon NAT," says Russ Housley, chairman of the Internet Engineering Task Force (IETF), the Internet standards body that created IPv6. "I hope this is not our future because it would be a very fragile Internet, making innovation more difficult. On the other hand, IPv6 will greatly reduce the need for NAT, restoring the opportunities for innovation that were envisioned by the original Internet architecture."

Dorian Kim, vice president of IP engineering, Global IP Network at NTT America, a leading provider of IPv6 services in the United States, says that without IPv6 the Internet "will be even more heavily NATed than it currently is, but life will mostly go on. Unfortunately, such an Internet likely will have a negative effect on potential development of application or service innovation due to inherent issues with NATs. Additionally, should service providers become more and more reliant on NATs, this will probably change the cost and scaling trajectories of Internet services over time due to high cost and limited scalability of large-scale NAT solutions.''

The upcoming World IPv6 Day is the biggest event in the history of IPv6, a 13-year-old standard of which the primary advantage over IPv4 is an expanded addressing scheme. While IPv4 uses 32-bit addresses and can support 4.3 billion devices connected directly to the Internet, IPv6 uses 128-bit addresses and can connect up a virtually unlimited number of devices: 2 to the 128th power.

The Internet needs IPv6 because it is running out of IPv4 address space. The free pool of unassigned IPv4 addresses expired in February, and in April the Asia Pacific region ran out of all but a few IPv4 addresses being held in reserve for startups. The American Registry for Internet Numbers (ARIN), which doles out IP addresses to network operators in North America, says it will deplete its supply of IPv4 addresses this fall.

But as necessary as IPv6 seems, there is a major stumbling block to its deployment: It's not backward compatible with IPv4. That means network and website operators have to upgrade their network equipment and software to support IPv6 traffic, and so far most have been unwilling to do so.

Despite all of the network industry momentum around World IPv6 Day, the protocol is not taking off on the Internet anywhere near as fast as proponents had hoped. A recent survey of Internet traffic compiled by Arbor Networks found that IPv6 represented less than 0.2 percent of all Internet traffic. Indeed, Arbor said IPv6 traffic -- both tunneled and native -- had declined 12 percent in the last six months, even as momentum for World IPv6 Day was building. Arbor gathered this data by surveying six carriers in North America and Europe.

Craig Labovitz, chief scientist of Arbor Networks, says the decline in IPv6 traffic is the result of users replacing inefficient IPv6-over-IPv4 tunnels with native IPv6 traffic, which he says is a sign of IPv6 becoming more production ready.

Although he concedes that the IPv6 migration effort has been unsuccessful to date, Labovitz said he is hoping that World IPv6 Day will change the protocol's momentum.

"My biggest hope for IPv6 Day is that the large content providers will gain enough confidence to leave IPv6 ... on by default,'' he says. "If this happens, we will have broken through the Catch-22 of subscribers/enterprises waiting for content, and content waiting for subscribers/enterprises to deploy IPv6."

If IPv6 adoption continues to lag, Labovitz warns that events may overtake the Internet engineering community.

"Instead of planned and well thought-out evolutionary Internet architecture, we end up with market forces creating a swamp of workarounds, hacks and other problems that add expense, stifle innovation and limit the potential of this fantastic global network," Labovitz predicts.

In contrast to the Arbor Networks data, NTT America reports increasing demand for the new protocol from its telecom, IT, hosting, government and education customers. NTT says 30 percent of its customer ports and 70 percent of its peering ports are now IPv6 enabled.

"We foresee a gradual, organic growth with IPv6 deployment among our customer base, especially as more and more become aware of the importance to transition," Kim says. "Companies that don't take action toward transitioning from IPv4 to IPv6 risk increased costs and limited functionality online for their users. Ultimately, however, IPv6 will result in faster, more secure, more reliable and cheaper Internet service."

Some experts say that IPv6 is not a sure thing, despite the efforts by the IETF, the Internet Society and other proponents to portray it as such.

Professor Milton Mueller, of the School of Information Studies at Syracuse University and a founder of the Internet Governance Project, points out that network operators can't switch to IPv6 today without cutting themselves off from the IPv4-based Internet. Instead, they have to run both IPv6 and IPv4 side-by-side in a dual-stack configuration or use NAT devices to bridge between the protocols. The awkwardness of this upgrade is why Mueller is not too optimistic about IPv6 deployment.

Five years from now, the Internet could still be "95 percent IPv4," he says. "I think it's possible, and I think it's bad. I can't think of anything better than having abundant address space in economic terms, but the migration strategy of dual-stack is a very strange thing."

Policymakers remain optimistic that IPv6 will eventually catch on because of demographics. The Internet has 2 billion users and is almost out of its IPv4 address supply, while the planet has another 4 billion people who aren't yet connected to the network. Proponents argue that IPv6 is the best path for the Internet to scale to meet the demands of these new users and their devices.

John Curran, president and CEO of ARIN, concedes that there has been little demand for IPv6 during the last decade.

"Nobody needs an oxygen mask on a plane until the plane is going down," Curran says. "The IPv6 need is now upon us, and it will have a significant growth rate over the next five years."

Curran says he's seen "a huge increase" in demand for IPv6 addresses from both ISPs and enterprises. For example, ARIN's IPv6 assignments to ISPs and enterprises rose more than 50 percent in 2010 vs. 2009, and they continued to climb in the first quarter of 2011.

"The number of organizations looking at IPv4 as a dead end is increasing," Curran says. "There's a lot of serious deployment. [Five years from now], IPv6 may still be a smaller amount of Internet traffic than IPv4. But you can't take 4 billion addresses and make them meet a 50 billion type of need."

Geoff Huston, chief scientist at APNIC, the Asia Pacific Network Information Centre, says the purpose of World IPv6 Day isn't so much to drive IPv6 traffic for 24 hours but to address concerns by content providers that IPv6 deployment is expensive and will result in their site being unreachable by a significant number of users.

"The point of the exercise is actually one of myth busting," Huston says. "In and of itself, [World IPv6 Day] won't get the IPv6 ball rolling, but it's an attempt to remove one more element of blockage, namely the fear that the problems caused by going to dual-stack will make the customer experience worse for a significant proportion of the current customer base."

Huston says the greatest risk if IPv6 doesn't get adopted widely is that the Internet will be closed to startups because all of the address space will be held by incumbent network operators.

"We are now at one of those massive crisis points in the evolution of the Internet," Huston says. "We can continue with ... an open Internet where incumbents invest in IPv6 in order to allow a common future for themselves and for future competitors to enter the market ... or we could gradually shut down the open network and build a network of islands of IPv4 with various toll gates and bridges that offer a small aperture to the outside world.''

Huston warns of a future similar to the 1960s and 1970s, when the telecom industry consisted of monopoly players that were slow to innovate. In order for the Internet to continue to support vibrant competition and radical innovation, he says, "IPv6 must not and cannot fail."